PTFE works very well for rotary shaft seals, but keep in mind that PTFE is available with various fillers that can enhance key properties for your application. In this blog post, we are going to talk about fillers for dynamic PTFE seals and what hardware is most appropriate for that filler.
Unfilled PTFE, also known as virgin PTFE, is recommended for use in applications without pressure on shafts with a hardness value of less than 30 HRC. Unfilled PTFE has an extremely low coefficient of friction. Note that it is FDA approved and has been found especially useful in cryogenic applications.
PTFE filled with molybdenum disulfide (aka, Moly) increases the wear resistance of PTFE without the same level of abrasiveness that that results from adding glass or carbon. It increases hardness, reduces friction, improves wear resistance, and reduces stick-slip behavior. It has pretty good chemical resistance, with its main weakness being strongly oxidizing acids. The associated hardware should have a surface hardness greater than 58 HRC.
Bronze-filled PTFE is used for high-speed applications involving hydraulic media. Because bronze is susceptible to chemical attack, it is not recommended for chemical applications. When molybdenum disulfide is included with the bronze filer, the result is excellent wear properties along with excellent extrusion resistance when used in high-pressure applications.
Glass-filled PTFE is recommended for general applications except those involving low-hardness shafts. The reason for this limitation is the hardness and abrasiveness that results because of the glass. The primary benefit of using glass as a filler lies in increased service life, enhanced strength, and better creep resistance. The addition of glass has a minimal effect on the chemical compatibility of PTFE but it should not be used in alkaline environments. Note that this grade of PTFE is available in FDA approved forms.
PTFE can be filled with both glass and molybdenum disulfide. This results in increased strength, a longer service live, better elongation properties, and less abrasion than PTFE with glass alone. Both glass-filled and glass-filled with Moly requires a shaft with a hardness greater than 58 HRC.
Carbon and Graphite
Carbon-filled PTFE includes both carbon fibers and graphite powder. The addition of carbon adds improved wear resistance and acts as a natural lubricant. It also decreases gas permeability, improves heat dissipation, and decreases creep. Carbon-filled PTFE can be used in steam and chemical applications in severe service environments that include high temperatures and elevated pressures (combined with lower speeds). The addition of carbon and graphite have very little impact on the excellent chemical compatibility that PTFE naturally possesses. Note that while it is abrasive, it is less abrasive than glass.
PTFE can include both carbon and molybdenum disulfide as additives. The result is a material that functions at very high temperatures where lubrication is not recommended. The recommended hardness is greater than 58 HRC.
In order to improve wear resistance and creep, other polymers such as polyimide can be added to PTFE in order to improve wear and creep characteristics. Polyimide in particular reduces friction to unbelievably low values, increases both wear and abrasion resistance, does not harm the mating surface, and works very well in dry running applications. The recommended hardness for the mating hardware is above 58 HRC.
There are a variety of fillers available for achieving optimum properties in a dynamic PTFE seal, including molybdenum disulfide, bronze, glass, carbon, graphite, and polyimide. When the correct fillers are matched with the appropriate hardware, the result is a powerful, dependable sealing solution.
Interested in reading more about PTFE filler materials?
See these blogs:
- Selecting the Right PTFE Filler for Your Application
- Fillers and Additives for Enhancing the Mechanical Properties of PTFE